“It says on your school list that you’re supposed to have dress robes this year . . . robes for formal occasions.” -- Molly Weasley (GF10)

Dress robes are a formal style of robe that witches and wizards wear for special occasions. At Hogwarts, students in their fourth year and above are required to have dress robes to wear to formal events. In the books, this is exemplified at the Yule Ball.

Descriptions of students in dress robes:

Harry: much like his school robes, but bottle green (bought for him by Molly using his money)

Ron: (bought second-hand by Molly, so she didn't have many choices; frightfully out of style) maroon with lace at the collar and cuffs, which he tried to remove with a Severing Charm.

Draco Malfoy - black velvet with a high collar, which in Harry's opinion made him look like a vicar

Pansy Parkinson: very frilly robes of pale pink

Hermione: floaty, periwinkle-blue material

McGonagall: red tartan and a rather ugly wreath of thistles around the brim of her hat

Ludo Bagman: bright purple with large yellow stars

Madame Maxime: flowing gown of lavender silk

Percy Weasley: brand-new, navy-blue dress robes

Stores where you can buy clothing/wizard dress robes:

Madam Malkin's Robes for All Occasions

During one summer sale, they sold robes which were spangled, self-ironing, beautifying, slimming, fattening, lengthening, temperature-adjusting, and plain, with a free frog-skin belt with every purchase (DP).